The present invention generally relates to image forming methods, and in particular to a method of forming an image on a recording medium by scanning an optical beam.
Conventionally, there is an image forming method wherein an image is written on a recording medium by scanning an optical beam that may be produced by a laser diode, LED array, FL dot-array, and the like. The scanning may be achieved either by using a raster-scanning system that deflects the optical beam such as a rotary polygonal mirror or a solid-state optical scanning head such as LED array, LCD shutter array or fluorescent dot array.
In any of the foregoing image forming systems, the optical beam produced by an optical source scans a charged photosensitive body in a horizontal scanning direction to form an electrostatic latent image, and this electrostatic latent image is developed by the toner. The image thus developed is then transferred on a recording sheet and fixed thereon.
In order to improve the quality of the image thus recorded, there are various proposals like the one described in the Japanese Laid-open Patent Application No.64-33574. This reference describes a system comprising memory means for storing image information of at least three scanning lines and control means for controlling at least one of the beam spot size and the beam intensity stepwise, based upon the content of the memory means. More specifically, the system controls one or both of the beam spot size and the beam intensity of a picture element that is surrounded by eight picture elements in correspondence to three scanning lines, in three to four steps based upon the image information of the eight surrounding picture elements.
Further, there is another image recording system proposed by the assignee of the present invention in the Japanese Laid-open Patent Application No.63-290752, wherein memory means for storing a plurality of picture elements including an aimed picture element and surrounding picture elements, is combined with discrimination means for discriminating whether the aimed picture element is an isolated white picture element or an isolated black picture element. Based upon the result of discrimination, the system changes the pulse width of the recording signal, and the beam size of the recording optical beam is changed based upon the recording signal.
In the former system, however, all eight picture elements have to be taken into consideration for the image processing, and there exists a problem in that a complex processing, too complex for use in an image recording apparatus, is required. The problem is deteriorated further because of the somewhat large number of steps in the stepwise control of the beam size or beam intensity. Such a complex processing inevitably increases the cost of the image recording system.
In the latter system, too, there exists the problem of too complex processing, as this system requires processing of at least five picture elements including the aimed picture element, two at both sides of the aimed picture element in the horizontal scanning direction and two at both sides of the aimed picture element in the vertical scanning direction.
Before starting the description of the present invention, the principle and problem of the image processing employed conventionally for improving the picture quality will be examined with reference to FIGS. 1(A)-1(I) and FIGS. 2(A)-2(I). These drawings show the examples of image patterns subjected to the image processing. In FIGS. 1(A)-1(I), the circles represent the position of the beam spot while the hatched region surrounded by the thick lines represents schematically the black, exposed part of the image. Thus, the drawings represent the typical image exposure process wherein the desired image is written by the optical beam that exposes the photosensitive body according to the desired image. On the other hand, FIGS. 2(A)-2(I) show the case of the background exposure process wherein the desired image corresponding to those of FIGS. 1(A)-1(I) is represented by the region that is not exposed by the optical beam.
Referring to FIGS. 1(A)-(I), FIG. 1(A) shows an isolated dot, FIG. 1(B) shows an array of isolated dots, FIG. 1(C) shows an oblique line of dots, FIG. 1(D) shows parallel oblique lines of dots, FIG. 1(E) shows a cross formed by intersecting lines of dots, FIG. 1(F) shows a vertical line of dots, FIG. 1(G) shows parallel vertical lines of dots, FIG. 1(H) shows a horizontal line of dots, and FIG. 1(I) shows parallel horizontal lines of dots. On the other hand, FIGS. 2(A)-2(I) show the counter part in the background exposure mode.
In the picture images of FIGS. 1(A)-1(I) and FIGS. 2(A)-2(I), some images, particularly those including oblique lines have problems that deteriorate the quality of the picture when recorded on a recording sheet as it is. For example, in the cross pattern of FIG. 1(E), there is a tendency that the node where the two lines cross each other tends to bulge in the recorded image because of the doubled exposure. When the size of the optical beam is reduced in correspondence to the node part to eliminate this problem, on the other hand, the node in the cross pattern of FIG. 2(E) tends to bulge. In the oblique line of FIG. 1(C), on the other hand, there arises a problem in that the line appears excessively thin because of the neck-shaped regions formed between adjacent dots.
In order to cope with these problems, conventional image recording systems have employed complex image processing as already described. When using such an image recording system for commercially competitive printers of personal computers or facsimile machines, a restriction is imposed such that the image processing employed should not be excessively complex.